The present disclosure relates to a rotary electric machine driving device that drives and controls an alternating-current rotary electric machine.
In recent years, from the viewpoint of, for example, energy saving and reduction of environmental loads, hybrid vehicles and electric vehicles each provided with a rotary electric machine as a source of driving force have been gaining attention. Such a vehicle includes a direct-current power supply, such as a battery, which supplies power when the rotary electric machine functions as the source of driving force (motor), and stores generated power when the rotary electric machine functions as a source of power (generator). When the rotary electric machine functions as the source of driving force (motor), direct-current power supplied from the direct-current power supply is converted into alternating-current power by an inverter to drive the rotary electric machine. When the rotary electric machine functions as the generator, alternating-current power generated by the rotary electric machine is converted into direct-current power by the inverter to be stored as regenerated power in the direct-current power supply.
A smoothing capacitor for smoothing the direct-current power is provided between the direct-current power supply and the inverter. In general, the inverter is constituted using switching elements, which are controlled to be switched at a predetermined switching frequency. As a result, a pulsation corresponding to the switching frequency is superposed on the direct-current power. The smoothing capacitor reduces fluctuations, such as the pulsation, of the direct-current power to keep voltages applied to the switching elements from exceeding the withstand voltage thereof, and keep the fluctuations in current from deteriorating the direct-current power supply.
In general, the direct-current side of the inverter for driving the rotary electric machine as the source of driving force of a hybrid electric vehicle or an electric vehicle is at a high voltage of 200 [V] to 400 [V]. Hence, the smoothing capacitor is required to have a high voltage resistance performance against such a high voltage, and at the same time, the fluctuation due to the pulsation needs to be considered. In addition, when withstand voltages of switching elements constituting the inverter are considered, the smoothing capacitor is required to have a sufficient capacitance for reducing the pulse component. These requirements generally increase the cost of the smoothing capacitor, and increase the physical size thereof, thus requiring a large installation space. Furthermore, the inverter and the smoothing capacitor are often installed in an integrated manner or mutually close to each other, as parts of a rotary electric machine driving device. In particular, an in-vehicle rotary electric machine driving device is required to be light in weight and small in size from the viewpoint of weight and installation space, so that the inverter and the smoothing capacitor are desired to be light in weight and small in size.
For example, Japanese Patent Application Publication No. 2009-106046 (JP 2009-106046 A) describes a space-saving rotary electric machine driving device (power converter) including a cooling mechanism. In this rotary electric machine driving device, a power module including switching elements is disposed on a flat surface inside of a case having a heat radiating portion. A smoothing capacitor electrically connected to the power module is disposed adjacent to the power module on a flat surface formed one step lower than the flat surface on which the power module is disposed (refer to paragraphs 7 and 8, FIG. 1, etc.). The smoothing capacitor required to have a high withstand voltage and large capacity tends to have a large physical size. In JP 2009-106046 A, the heat radiating portion and the power module are disposed in accordance with the height of the smoothing capacitor such that the overall height of the rotary electric machine driving device is reduced, thereby saving space.
In this manner, space saving can be achieved to some extent by making improvements in the layout of, for example, the circuit portion (power module), the cooling mechanism, and the smoothing capacitor of the inverter. However, reduction of weight and size of the entire device has not been accomplished by further reducing the physical size of the smoothing capacitor determined by the withstand voltage and the capacitance. Reducing the capacity of the smoothing capacitor allows the size to be reduced, but may reduce the effect of suppressing the deterioration of the direct-current power supply and the switching elements. Specifically, reducing the capacity of the smoothing capacitor to reduce the size thereof lowers the capability of smoothing the direct-current power, and thus increases the pulsations of a system voltage that is the voltage on the direct-current side of the inverter and a power supply current that is a current flowing through the direct-current power supply. This results in the possible reduction in the effect of suppressing the deterioration of the direct-current power supply and the switching elements.